Not great theatregoers, you might think, the members of the BNP, any more than our prime minister, who advertises a preference for the work of Simon Cowell. But you never know. One party dignitary, Richard Barnbrook, was a conceptual artist who acted in his own poetical film before he began a Wagnerian journey that has ended with his representing the BNP in the London assembly. If art-loving Barnbrook is representative of the membership, then perhaps it was understandable for Dudley council to worry that a performance of a new play that challenges white extremist thinking could jeopardise morale in their town, where the BNP has 10% of the vote.

The council became so worried, in fact, that it cancelled the play. Moonfleece by Philip Ridley, should have appeared at a theatre within its Dormston Sports & Arts Centre last Thursday. Instead, the director of the centre wrote to the producer of Moonfleece, Will Young, saying: "It has been brought to the theatre's attention that the play you are intending to show includes characters and themes of a political and potentially discriminatory nature. In view of this, the theatre/centre's governors and senior staff have taken the decision that the play and the theatre/centre's association with the play are unsuitable."

Dudley council appears unwilling to elaborate on these objections to the touring play which, though its story is too complex to rehearse here, has appeared without incident in Doncaster, Bradford and Leicester. Will Young says his company, which is now bringing the play to London, chose venues where its issues of race and homophobia were likely to resonate with audiences. At least in Dudley, where the English Defence League was free to march yesterday, he has been taken seriously. For it is, of course, one of the great cruelties of political theatre that its hard-hitting messages generally bounce off audiences who pride themselves on an ability to tolerate any amount of savage contempt for their bourgeois complacency and still enjoy a glass of white in the interval.

The more relentlessly instructive, the merrier. How else to explain the success of that dreary old sermon, An Inspector Calls? Give him a smelly lady in a van, such as would normally require an emergency call, or an anarchic drunk, like Jerusalem's Johnny Byron, and the cultivated theatregoer's happiness is complete. Even in hard, very real Dudley, one imagines that an evening audience in the Dormston arts centre would have been composed by those least likely to double as pillars of – or double agents for – the local BNP.

The council's belief that Moonfleece was too dangerous to be seen is so completely nonsensical that it seems, at first, overdramatic to relate this misguided ban to a wider, increasingly sinister willingness to suppress anything that might cause trouble. Yet it is hard to believe Dudley's worthies would have gone so readily for censorship had it not been for a climate in which new legislation on hate speech, supported by a host of amateur censors and protesters, constantly reminds everyone that free expression is tempered by a sacred duty not to offend other people.

That there appear to have been no local complaints in anticipation of Moonfleece, which transgressed no hate speech laws, only makes the Dudley case more abject. Like the publishers who shunned Orwell's Animal Farm because it mocked pro-Stalin orthodoxies ("The choice of pigs as the ruling caste will no doubt give offence to many people and particularly to anyone who is a bit touchy") or, more recently, the non-publication of the Danish cartoons, the Dudley ban has a group of self-appointed censors pre-emptively shielding civilians from material it judges them too unenlightened to deal with.

To an astonishing degree, considering its obvious risks, this kind of benevolent police-work is now flourished as a token of personal decency. For some on the left, progressiveness is denoted by the denial of platforms for one or more of the following: the BNP, Islamists, Israeli academics, climate change deniers, arrogant BBC comedians, newspaper columnists pushing their idea of "free speech" that bit too far. Last week, the progressive website Pickled Politics was enjoying the humbling of its current bête noir: "[Rod] Liddle doesn't believe in free speech," declared Sunny Hundal, "he simply believes in his right to say what he wants without regard for facts or any blowback." Blowback? As in a critical response to one's opinions? If so, it seems Tory commentators are apt to be equally heedless. At ConservativeHome, Tim Montgomerie has counselled offenders that "there is constructive criticism and there is destructive criticism. There is a time for debate on the right and a time to either be silent or gun for Labour".

God knows what reassures such speakers that their high-minded support for gagging has absolutely nothing in common with, say, that of the Chinese communist party and, moreover, that they will never suffer the consequences of their own selective approach to free expression. Already those Muslim Britons whose burning of The Satanic Verses began the vogue for pro-victim censorship have seen their co-religionists imprisoned for writing bad poetry or waving nasty placards. Ken Livingstone, the ex-mayor of London who wanted a public platform for his chum, a homophobic advocate of wife-beating, could not prevent the BNP being offered the same privilege on Question Time. The same Christians who now demand free speech for the American missionary to these shores, Shawn Holes (who has just been fined £1,000 for "homophobic remarks", ie, preaching the Bible), remain furious that they never got to destroy Jerry Springer: The Opera.

Now, with the ban on Moonfleece, supporters of Stonewall, which endorsed Straw's extension of hate speech to gay people, will be deploring Dudley council's suppression of a play which questions homophobic behaviour. "These people don't see," Orwell wrote, when he castigated self-censoring intellectuals, "that if you encourage totalitarian methods, the time may come when they will be used against you instead of for you."

It will be clear to censors of a liberal persuasion that the ban on Moonfleece is a travesty. The council has it the wrong way around. The whole point of official speech regulation is that, in its high-minded partiality, it selects only certain, vulnerable communities for protection. Which is one reason why the admirable Peter Tatchell, who has suffered more from free speech than most, has consistently opposed its restriction: "Everyone should be equal before the law," he wrote in 2007, when Straw added incitement to homophobic hatred to the 3,000 new crimes Labour had introduced since 1997. When Holes was convicted last week, for saying homosexuals would go to hell, the homosexual Tatchell condemned his treatment. "If I had known about this prosecution in advance, I would have gone to court to defend Mr Holes's right to freedom of expression," he said. "The price of freedom of speech is that we sometimes have to put up with opinions that are objectionable and offensive." How astonishing that it still needs saying.